Chorioamnionitis or inflammation in the fetal membranes is present in about 70% of the preterm infants born before 30 weeks gestation and is a major contributor to the morbidity and mortality in this population. Fetal inflammatory response syndrome (FIRS) is present in about 50% of preterm infants exposed to chorioamnionitis and thought to be the mediator of chorioamnionitis induced fetal injury responses. Despite many epidemiological studies implicating FIRS as a strong risk factor for adverse neurological, gastrointestinal and pulmonary outcomes in preterm neonates, very little is known about the mechanism or the pathogenesis of FIRS. A major stumbling block in the progress of understanding how FIRS is initiated or signaled is the lack of appropriate animal models. This proposal will test the hypothesis that fetal exposure to chorioamniontis induces a systemic inflammatory response that is initiated by IL-1 signaling in the amniotic compartment and is modulated by systemic innate immune responses. The experiments will utilize the preterm fetal sheep model of chorioamniontis induced by intraamniotic endotoxin developed by our group that closely mimics human pathology. This application is based on our preliminary data that IL-1 signaling is required to mediate intraamniotic endotoxin induced lung inflammatory responses and that the sheep fetus adapts to repeated exposures to intraamniotic endotoxin with decreased monocyte responsiveness to endotoxin. In Specific Aim 1 we will define the relative contributions of the chorioamnion, lung and gastrointestinal tract to development of FIRS induced by endotoxin. We will utilize fetal surgical methods developed by our group to isolate each of the organs. In Specific Aim 2 we will test if intraamniotic IL-1 causes FIRS and determine the requirement of IL-1 signaling in LPS induced FIRS using an IL-1 receptor antagonist. In Specific Aim 3 we will evaluate fetal adaptation and endotoxin tolerance to repeated exposures of intraamniotic endotoxin. Our long term goals are to understand the mechanisms of systemic inflammation and organ injury responses following exposure to chorioamnionitis. This grant will provide a framework to understand the problem of FIRS at a whole animal level. This work will provide insights necessary to the development of therapeutic strategies such as selective intraamniotic anti-inflammatory therapy to improve outcomes in preterm infants exposed to chorioamnionitis.